1. Technical Field of the Invention
The present invention relates to a voltage monitoring device, and in particular, to a voltage monitoring device using a flying capacitor method, which monitors voltage of each of battery cells connected in series to one another to configure an assembled battery by using a plurality of capacitors.
2. Related Art
In related art, a device using a capacitor is proposed as a voltage monitoring device that monitors voltage of each of battery cells connected in series to one another to configure an assembled battery, e.g., an in-vehicle high voltage battery mounted in vehicles such as hybrid vehicles or electric vehicles. This device is also called a voltage monitoring device using a flying capacitor method or a flying capacitor type voltage monitoring device.
For example, JP-A-2002-289263 discloses a voltage monitoring device using a flying capacitor method, which detects voltages of two adjacent battery cells in the assembled battery by using a pair of capacitors connected in series to each other. This flying capacitor method using a pair of capacitors is also called as a double flying capacitor method.
The device using the flying capacitor method is provided with a resistor functioning as a filter between each of the battery cells and the capacitor. Depending on a resistance value of the resistor and a capacitance value of the capacitor, a time (time constant) required for the capacitor to be charged with voltage of the battery cell varies.
In the device using the double capacitor method disclosed in JP-A-2002-289263, the pair of capacitors are connected in series to each other, and therefore, total capacitance of the capacitors becomes smaller in charging the capacitors with voltage of the battery cell. This can shorten a time for required for the capacitor to be charged by voltage of the battery cell, when the pair of capacitors are used for detecting voltage of the battery cell, compared to when only one capacitor is used for detecting voltage of the battery cell.
Additionally, in the double capacitor method, each of the capacitors is charged with voltage of the same battery cell to compare charged voltage of each of the capacitors with each other, thereby being able to detect, e.g., disconnection of detection lines which connect each of the capacitors and each of the battery cells.
On the other hand, the device using the double capacitor method disclosed in JP-A-2002-289263 may detect voltage of the battery cell by using not only both of the capacitors but also one thereof.
For example, when voltage of the battery cell is detected by using the pair of capacitors, current does not flow in a detection line connected to a connection point between each of capacitors among a plurality of detection lines connecting each of the capacitors and each of the battery cells. In this case, if this detection line is disconnected, this disconnection cannot be detected. Due to this, voltages of the battery cells may be detected individually by using one of the pair of capacitors.
However, in using one of the pair of capacitors, if a resistance value of each of the plurality of resistors functioning as the filter are the same value, total capacitance of the capacitors becomes larger compared to using both of the pair of capacitors. This results in an increase in a time for required for the capacitor to be charged with voltage of the battery cell.
As one resolution to shorten a time for required for the capacitor to be charged with voltage of the battery cell in using one of the pair of capacitors, all of the resistors functioning as the filter may have the same resistance value. In this case, a filter function of each of the resistors may be damaged.
The same issue explained above may occur in a voltage monitoring device using a flying capacitor method which is configured to detect voltage of each of a plurality of battery cells using three or more capacitors.